The structure of a chemical compound determines its physical behavior, including melting point, hardness, and electrical properties. When examining Magnesium Oxide (MgO), the key question is whether the atoms are held together by an ionic or a covalent bond. Understanding this connection is fundamental to predicting how the substance will behave. The bond classification depends primarily on the inherent characteristics of the atoms involved.
Defining Chemical Bonds
Chemical bonds form when atoms interact to achieve a more stable electron configuration, typically resembling that of a noble gas. Bonds are generally classified into two main types: ionic and covalent. An ionic bond involves the complete transfer of one or more valence electrons from one atom to another. This transfer results in the formation of positively charged ions (cations) and negatively charged ions (anions). The resulting strong electrostatic attraction between these oppositely charged ions holds the compound together, typically observed between a metal and a nonmetal.
Covalent bonds, in contrast, form when atoms share valence electrons rather than transferring them. This sharing results in the formation of distinct molecules and usually occurs between two nonmetal atoms. The key factor for determining the type of bond is the difference in electronegativity between the two atoms. Electronegativity measures an atom’s ability to attract a shared electron pair toward itself. A large difference in this value leads to the complete transfer of electrons and the formation of an ionic bond.
The Elements Involved: Magnesium and Oxygen
Magnesium Oxide is formed from the elements Magnesium (Mg) and Oxygen (O), each possessing distinct chemical tendencies. Magnesium is a metal found in Group 2 of the periodic table, known as an alkaline earth metal. Having two valence electrons, Magnesium readily loses these electrons to achieve the stable electron configuration of the nearest noble gas, becoming a cation with a 2+ charge (\(\text{Mg}^{2+}\)). This tendency is reflected in its relatively low electronegativity value, which is approximately 1.31 on the Pauling scale.
Oxygen is a nonmetal located in Group 16, and it requires two additional electrons to fill its outermost shell. It has a strong tendency to gain two electrons, forming an anion with a 2- charge (\(\text{O}^{2-}\)), thereby achieving a stable configuration. This strong electron-attracting ability gives Oxygen a high electronegativity value of approximately 3.44. The difference in these values is calculated as \(3.44 – 1.31\), which equals 2.13.
This significant difference in electron-attracting power is a strong indicator of the bond type that will form. The combination of a metal and a nonmetal, coupled with their positions on opposite sides of the periodic table, supports the prediction of a bond involving electron transfer.
Determining the Bond in Magnesium Oxide
Based on the analysis of the constituent atoms and the calculated electronegativity difference, the bond in Magnesium Oxide is classified as predominantly ionic. The difference in electronegativity (2.13) far exceeds the commonly accepted threshold of 1.7 to 2.0. This threshold is generally used to distinguish between a polar covalent and a predominantly ionic bond.
The formation of MgO involves the two valence electrons from the Magnesium atom being transferred to the Oxygen atom. This process creates the \(\text{Mg}^{2+}\) cation and the \(\text{O}^{2-}\) anion, which are then held together by powerful electrostatic forces. The resulting structure is not a simple molecule but a crystal lattice, where countless ions arrange themselves in a repeating, three-dimensional pattern.
This strong ionic bonding explains the physical properties of Magnesium Oxide. The compound is a hard, brittle solid at room temperature and possesses a high melting point, typically around 2852°C. These characteristics are consistent with the energy required to break the strong electrostatic attraction within the ionic crystal lattice. Like many ionic solids, Magnesium Oxide is a poor conductor of electricity in its solid state because the electrons are fixed in the strong bonds.